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A previous study, looking at the creep-rupture behaviour of mixed reinforcement GRP when immersed in water, had discovered that low loads, behaviour became temperature sensitive. Since the recorded time to failure of a sample was reduced at elevated temperatures, from that predicted by a linear extrapolation of the short term creep-rupture results, this deviation caused problems in the accurate prediction of long-term design stresses. In order to improve the accuracy of long term design predictions, it was decided to study the mechanisms of creep in GRP that initiates time dependent failure. From this, it was hoped that accurate design criteria suitable for predicting GRP response over a 30 year design life from short term creep tests, could be developed. This thesis reports the results obtained from such a study. A series of creep tests were performed on mixed reinforcement GRP samples at several stress levels, both in air, and in room temperature distilled water, using a microcomputer based data collection system. In conjunction with this work, damage development in samples, due to combinations of water uptake and creep loading, was followed, using both scanning electron, and optical, microscopy. Moisture uptake measurements were undertaken under a series of load/temperature regimes, and fibre/matrix debonding followed using photographic techniques. In this way, water absorption, both in terms of uptake rate, and location within a sample, could be characterised. Tensile tests were also performed to determine the standard mechanical properties of the mixed reinforcement GRP used. It was found that a critical damage state was created at loads in excess of 50% of ultimate, but not below. This took the form of between 2 and 8 neighbouring filament breaks in the longitudinal woven rovings at weave crossover points, producing microcracks in the reinforcement. The creation of this multifilament fracture damage during primary creep, was considered to be necessary for time dependent failure to occur in air. Secondary greep strain was found to increase in discrete steps, both in air and water. This was attributed to the formation of transverse grasks in the longitudinal woven rovings, propogating from the above critical damage. In water, diffusion was found to be non-Fickian. Moisture uptake increased with increases in applied load and temperature. Water was seen to accumulate at weave cross-over points when immersed under load. This led to stress-enhanced fibre corrosion in these regions, weakening the reinforcement, and reducing the failure time from that expected at the same load level in air. The localised nature of moisture degradation was thought to result in the formation of critical fibre damage at loads below 50% of ultimate, when immersed in water. Two design criteria based on the observed creep mechanisms, have been developed for GRP that predict response when loaded in either air, or water. Both predict the existence of creep-rupture limits at low loads.

The research reported in this thesis deals mainly with the use of novel nanotechnology-based testing methods in the field of cement-based composites. The existing knowledge of indentation test methods is presented and reviewed. The research presented focuses on the development and pilot usage of depth-sensing indentation (DSI) test methods. The use of DSI test methods for cement-based materials covers two distinct areas. The first area includes the testing of micromechanical properties of cement pastes/matrices. The development in DSI test methods allows direct measurements of properties, such as hardness, elastic modulus, etc., at microscale. Special attention is paid to assessment of interfacial regions in such cement-based materials. In the second area, DSI test methods are used for assessment of interfacial properties of fibre reinforced cementitious composites, with focus being directed to composites reinforced by bundles of microfilaments. A new push-out test method for individual microfilaments collated in a bundle and embedded in cementitious matrix is proposed and developed. Novel use of other nanotechnology-based techniques, such as focused ion beam (FIB) techniques, forms another part of this thesis. The focused ion beam milling technique was utilised for production of diamond probes which enabled push-out tests of individual glass microfibres to be carried out. Also, FIB cross-sectioning of indents induced by DSI test methods was performed. This novel research method showed large potential for a better interpretation of the test and an improved understanding of the microfracture processes in cement-based materials. Detailed information about FIB techniques is therefore presented in a separate chapter. The focus of this project has been to develop methods which will enable further systematic research into micromechanical properties of cementitious materials and may lead to the ultimate goal of this investigation - the development of a new generation of materials of improved macromechanical properties and durability.

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